Fatty acid esters usually contain sulfur in a content of at least several milligrams/kg to several tens of milligrams/kg. Here, the fatty acid ester means esters of fatty acids and glycerin (triglycerides, diglycerides and monoglycerides) or esters of fatty acids and alcohols having 1 to 22 carbon atoms (fatty acid alcohol esters) and partly contains free fatty acid depending on the case. When these fatty acid esters are allowed to undergo a hydrogenation reaction in the presence of an ester reduction catalyst to produce alcohols, sulfur contained in the fatty acid esters acts as catalyst poison of a hydrogenation catalyst to significantly deteriorate the catalyst activity.
In the case of, particularly, a fixed-bed continuous reaction, the life of the catalyst is very shortened, which requires frequent exchanges of the catalyst, so that a reduction in the rate of operation is unavoidable.
Also, the fatty acid ester is known as a light gas oil substitute used for bio-diesel fuel and there is demand for fatty acid esters reduced in the content of sulfur in such an application from the viewpoint of reducing sulfur oxides in exhaust gas.
Therefore, various studies have been made so far with the intention of removing sulfur which is to be a catalyst poison of hydrogenation catalysts. For example, methods reducing the content of sulfur by distillation are frequently used because sulfur compounds have relatively higher boiling points. However, it is impossible to remove all sulfur compounds by distillation. Also, in order to decrease the concentration of sulfur to a level as low as 0.5 mg/kg, it is necessary to dispose of a large amount of originally necessary high-boiling components with a remarkable reduction in yield.
Also, in petrochemical fields, a desulfurization catalyst containing, as major components, molybdenum, cobalt and nickel is used. In order to make these catalysts develop desulfurization activity, a reaction temperature of 300° C. or more is necessary. In order to improve desulfurization activity, an attempt is made to define the pore volume having a pore diameter of 6 to 9 nm or 7 to 12 nm (JP-A 2000-42413 and JP-A 2000-79343). The reaction temperature should be 300° C. or more.
In the meantime, a method is disclosed in which a sulfur compound is hydrocracked in a hydrogen atmosphere to adsorb the sulfur compound by using an adsorbent containing a metal such as Ni or Cu (JP-A 5-978 and JP-A 5-57286).
Also, JP-A 2002-523230 discloses a nickel catalyst which contains 87.5 to 99.9 percent by weight of nickel and may be used for desulfurization of a solvent.